Our previous studies have provided a proof-of-principle that a small disulfide-rich miniprotein can be developed that will block the infection of cells by Langat (LGT) virus, a naturally attenuated virus that is a model for the pathogenic members of the tick-borne encephalitis (TBE) serogroup of the Flavivirus genus. A first generation miniprotein, termed MP-100, was selected by panning a conformationally restrained combinatorial miniprotein phage display library for binding with purified recombinant domain III (D3) of the LGT virus envelope (E) protein. The miniprotein MP-100 was shown to block infection of Vero and LLC-MK2 monkey kidney cell cultures by tick-borne LGT and Powassan viruses. Further studies indicated an antiviral effect in a mouse animal model. Our objective during this period of support is the development of a second-generation, more tightly binding miniprotein with improved antiviral activity against TBE serogroup flaviviruses compared to the current MP-100 sequence. Our goal is to develop an antiviral miniprotein that is effective against a broad range of potential flavivirus bioterrorist threat agents in the TBE serogroup, including Central European tick-borne encephalitis (strain Kumlinge), Kyasanur Forest Disease (FD), Omsk Hemorrhagic Fever (OHF) and Russian Spring Summer encephalitis (RSSE) viruses. We will identify optimized tight-binding analogs of MP-100 to OHF-E-D3 and the related TBE serogroup E-D3s, and determine if they have enhanced antiviral activity in tissue culture cells and in animals. The development of anti-TBE virus miniproteins will serve as a model for the development of miniproteins against other flaviviruses that are also potential bioterrorist threat agents or emerging diseases, including dengue, Japanese encephalitis and West Nile.